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Could a turbofan have two adjacent fans?
Is there any a point where it becomes possible/ideal to run two large front fans (side by side) powered from a 'single' core? Otherwise shoot this to pieces and explain the ways this is a bad idea.
This inspiration for this question is from the design of some rocket motors which utilize a single turbine driving both pumps on a single shaft to feed multiple combustion chambers and nozzles.  Example would be the RD-180.

Partially related, aerodynamic issues with engines side by side

The GE Propfan has two counter-rotating fans, but they're not inside the nacelle like a fanjet

@fooot I had considered the points mentioned, however in the case I have presented the following differences are apparent.  There aren't two engines, the size of the fans and physical configuration with relation to the proposed core is also open to speculation.

are you planning on running a gearbox to split the single shaft to the fans? Gearboxes are ... a challenge! Ask any Chinook engineer.

@RoryAlsop: And yet we have geared turbofans...

Actually, this is getting more and more attention lately. Only, the core would drive a generator, the generated electricity would then be distributed to different fans (2 or more). I could compose an answer if this fits your question description?

Considering our progress on gear boxes, we are at least 20 years away to make this practical. But, this is actually a good solution to the 737 problem.

Generally, it is more efficient to have one large than two small devices.
The twin fan solution not only needs a gearbox and driveshafts that a single fan solution does without, it also has more intake and nozzle surface area per cross section than the single fan. This will cause more viscous losses and lower efficiency. The only advantage would be if size limitations make a single fan impossible, but in that case it would be more straightforward to have each fan driven by its own core engine.
Famous examples of aircraft which used a single engine and two "fans" would start with the Wright Flyer models, all of which had a single engine drive two propellers via bicycle chains. But the list is short.
The list of opposite designs where two engines would drive a single propeller or fan would probably be longer, and my favourite from this list is the LearFan.
@jwenting correctly reminds me that for completeness a single power plant driving two contra-rotating propellers should also be mentioned. This is indeed the only way on airplanes of one engine driving two propellers, albeit not adjacent, that has seen wider adoption, from the RR Griffon on the Avro Shackleton to the NK-12 of the AN-22, Tu-95 and Tu-114.
Of course, all single-engine helicopters also use this one engine to drive their two propellers …

A splitter gear on the shaft to run a second fan, surely must weigh less than a full engine core, minus the large amounts of ancillary systems required to run a  full engine.

@jCisco: Still, the one large engine is almost as heavy as two small ones. Add gear and driveshafts, and the mass advantage is lost. Also, the systems on the large engine must be duplicated for redundancy while they can be single on the two small engines.

You forgot one geometry that has been successfully used and that is 1 engine driving 2 propellors in line (usually contra rotating ones). Not relevant to the question at hand, but for completeness it should be mentioned I think.

Actually, there's one example already: F35B STOVL.
It's not so impossible I guess. When the efficiency and weight penalty is offset by some other requirements, e.g. ground clearance (next gen 737-Max?), we may soon see a dual-fan-single-core layout in the future!
(F135 engine + Rolls-Royce LiftSystem)

From what I've read, bigger props (and fans) are more efficient, because they move more air. However, at high subsonic speeds, the tips of props start to run into the sound barrier.
So, that seems to argue for more smaller props/fans, whose ends won't hit the sound barrier.
As others have mentioned, extra crankshafts and gears cost weight. I think I've seen designs using superconducting generators and motors driving multiple fans. However, I can't find it at the moment, and superconductors are probably not ready for prime time...
I don't know if you could bleed air from the edges of one fan, to drive, say, a fan on each side. But if you could, that would move more air. Although it might weigh a lot.
This is all assuming one turbine has enough oomph to drive multiple fans. I don't know if that's the case, or will be any time soon.
Disclaimer: I'm guessing, from limited reading.

You may add reference you read. Don't hesitate to see the help center to see how you can improve your question.

Mechanically it is difficult, although moving more air with lower rpm props has the possibility of improving thrust efficiency per horsepower.
Keep in mind, while this approach certainly benefited the Wrights 30 to 50 mph craft, slower rpm props will more rapidly lose thrust efficiency as forward speed increases due to changes in their relative wind (Angle of Attack).  Especially for fixed props, smaller, with higher rpm, gave a greater effective thrusting speed range as aircraft design moved beyond the Wrights.  Variable pitch props partially solved this issue, enabling another leap forward to higher speeds, but now power output became more of an issue as 2x speed meant 4x drag leading to:
Multiple engines turning multiple props, which also improved safety in case one engine failed.
Further improvements in power output lead to:  more blades on props.
Still more power from jet turbines:  many bladed fans.
Both with lower thrusting efficiency but much more thrust output and greater speed range than 2 blade props.
So, could a turbo fan have 2 adjacent fans?  Increases in reliability and greater power producing efficiency certainly has started a trend towards fewer, larger jet engines with bigger fans.  One giant turbine running 2 fans?  Possible.
One giant turbine generating electricity for 4 fans? Aha!  No mechanical linkages needed!
But, especially for large passenger aircraft, going with less than 2 power plants will be unlikely in the near future, unless it's a zeppelin.


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How is the ATC language structured?
Is the content of spoken communications used by air traffic control and pilots defined as part of a rigorous language definition? Such as a morphological or lexical foundational system, or is phraseology the extent of formalization?
And with the advent of text/computer based communications and automation on the horizon so to speak, what is the state of formalizing a language of communication? (If not already answered in the first part of my question.)
References to white papers and other research is fine if the answer requires you to almost write one.

I don't think ATC phraseology is formalised in something like Backus-Naur notation, if that's what you are asking. Also, in real life, Pilot-ATC communications vary greatly from aviation authority prescriptions.

@RedGrittyBrick Chomsky based systems like BNF and other related notations aren't likely to work with the natural language aspect of ATC communications.  I was hoping there was some guiding document be it based in a weak BNF or a more modern linguistic framework. But if not that is is much of an answer as yes .

This is a promising question. There are at least rules for using verbs, adverbs, and there are ways to convey urgency, etc. I believe some information can be extracted from ATC manuals and pilot-ATC phraseology guides. Also glossaries.

See also: Defining a grammar of radio telephony and emergencies

Voice communication is going to become less and less of a thing with the implementation of Nexgen in the US. It won't disappear completely, but many vital pieces of ATC information will be exchanged with the aircraft electronically. The rest will be more like clarifications, changes/amendments, and untowered operations.

"Chomsky based systems [...] aren't likely to work with the natural language aspect of ATC communications" For ATC I don't know, but natural language is indeed the base of Chomsky's research on syntax. It has been used to simplify understanding of natural language by cognitive computers.

Yes that is right, however in practice the BNF he suggested is CFG applicable and for natural languages RTN likely a far more palatable and accessible method to both discuss, develop and likely publish.  Purely synthetic grammars such as BNF will do as a compact replacement. Likely to please only the computers, humans not so much, its a trip into the utterly abstract, kinda far from where I was hoping to land if some documentation did exist to answer my question.

I don't know about a formal structure, but ground to air and air to ground communications usually follow the 3-W or the 4-W rule.
For air to ground communication - use the 4-W rule:

Who the controller is (identification of the controlling agency)
Who you are (Aircraft Identification)
Where you are (what is you position and altitude)
What do you want (state your intentions or request)

Example:
Grand Junction Approach, Cessna one seven two Sierra Papa, five miles southwest of the ABC VORTAC at five thousand, five hundred, inbound for Grand Junction to land with ATIS Kilo.
Here the pilot identifies himself as a Cessna with the identification N172SP, that he is five miles southwest of the ABC VORTAC at 5,500 ft MSL, and that he intends to land at Grand Junction airport and has the latest ATIS weather report for Grand Junction.
ARTCC and other air traffic controllers will generally use the 3-Ws to communicate with you

Who you are (the aircraft they are giving their instructions to)
Who they are (the name of the controlling agency)
What they want you to do (instructions for the aircraft in question to follow)

Example:
Cessna one seven two Sierra Papa, Grand Junction Approach, turn right to 030.  Descent and maintain three thousand two hundred.  Expect vectors for runway three zero left.  Squawk six four one one and ident.
Here the controller (Grand Junction approach) has advised Cessna 172SP to turn to a heading of 030 and to descend to 3,200 ft MSL.  He should also expect further heading instructions for insertion into traffic for runway 30L at Grand Junction airport.  The controller also tells the pilot to set his transponder to the code 6411 and ident, so the controller can verify his position.
As a courtesy, and often requested or implied by controllers to verify the pilot in question heard the instructions and will comply, the pilot will read back the instruction in full to the controller, followed by his/her callsign.
Example:
Right to 030, descend and maintain three thousand two hundred, expect vectors for runway three zero left, squawk six four one one and ident, Cessna one seven two Sierra Papa.
The NATO phonetic alphabet will be used for individual letters to avoid confusion by both parties as many letters can sound similar on a garbled radio transmission. Other oddities include the use of the word 'niner' in place of the number 9; This is to distinguish from the number 5, which is pronouced "fife".
English is also the accepted international language for use in flying and radio communications, but is only required for flights under instrument flight rules (IFR).

While tough to pick out with the ear, pilots should be saying the word "fife" instead of "five", and "tree" instead of "three". You are also supposed to pronounce "four" as "fow er".

This answer covers the protocol of exchange, not quite what I was after, though I appreciate the info. Indeed its a great answer to the question:  What is the protocol of ATC communications

@RonBeyer Yeah, that's technically the standard, though a lot of native English speakers ignore it, especially when not operating in airspace that is likely to have aircraft with non-native English-speaking pilots. Even at busy international airports, though, numbers will very frequently be pronounced normally rather than by digit (e.g. "Delta Fifteen Seventy-Four" instead of "Delta One Five Seven Four".) Use of 'Niner' is pretty universal, but tree/fower/fife is less so in practice.

@reirab Completely agree, the NATO alphabet says what they "should" be saying, however what they actually say can be quite a bit different. I confess to only using niner myself.

@RonBeyer I find strange alternatives of the NATO alphabet quite strange, not only for the redundancy of their existence but total lack of consistency.  Singapore companies tend to use city names, which leads to S becoming San Francisco, Soul and even Singapore, much to the whim of whichever name they recall first.

@RonBeyer A few years back I flew a club plane N9999U.  After my extended "November niner niner niner niner uniform" callup, tower would shorten it to "triple niner uniform".

@Gerry Those kinds of things are allowed by JO 7110.65W ATC Phraseology Order, Section 2-4-17 and 2-4-20.

The FAA AIM contains some examples of standard ATC communication in "Section 2. Radio Communications Phraseology
and Techniques" (page 4-2-1).
Also, the gory details of what every word/phrase used in aviation means are defined in the "Pilot/Controller Glossary" at the end of the AIM
https://www.faa.gov/air_traffic/publications/media/aim.pdf

@Gerry if you were flying N9999U, why did tower refer to you as triple niner uniform instead of niner triple niner uniform or quadruple niner uniform? :)

@FreeMan I don't know. I never asked. It always seemed strange as everywhere else 'shortened' call signs were always the last 3, e.g.; zero eight six or zero nine yankee. I would have expected niner niner unifom, but tower always used triple niner uniform. Maybe they just found it easier to say. In any case, there was never any confusion about who they were talking to.

I think "niner" is to avoid confusion with "nein" (German for "no"), not with "five", and that "fife" and "fower" are to avoid confusion with "fire"?

No, aviation phraseology is not part of a rigorous language definition. When learning to speak on the radio, we do not learn specific sentence structures and grammar. Instead, phraseology is defined as a set of standard phrases, which we have to learn by heart and use appropriately. Although these phrases are based on the English language, they do not necessarily follow normal grammar rules, because they have been designed to be as short and unambiguous as possible.
With regards to the advance of electronic communication between ATC and cockpits, standard CPDLC (controller pilot datalink communications) messages have been established. These are based on the spoken phraseology, and are often an exact copy. You can get an idea from the image below of how CPDLC messages are structured. The first column displays the intended message, the second column indicates how it is actually transmitted.

(excerpt from PANS-ATM Appendix 5)

Yes. For ATC, clearance order is formalized in FAA Order 7110.65 in Chapter 4-2-1 "Clearances". It outlines specifically how clearances are to be issued, and the order in which the individual elements are to be issued. Clearance Items
In addition to this, phraseology examples in the 7110.65 are to be regarded as standard phraseology, and are to be used when such an example is given, eg, for "do this speed or SLOWER, November tree six fife Tango, do not exceed two five zero knots is correct, while the often-used November tree six fife Tango, Maintain two five zero knots or less is incorrect.
This is to give the verbiage a definitive difference from November tree six fife Tango, maintain two eight zero knots or greater, which is correct for "do this speed or FASTER"
There are also specific formats for issuing traffic advisories, weather information, airport conditions, pretty much anything a controller would have to issue as information or a directive.
Plain English is approved for use in situations where prescribed phraseology does not exist, or to aid in eliminating misunderstandings.
When obtaining information from a previously unknown aircraft, the four Ws format who are you, where are you, what do you want, and sometimes why do you want it.
With all that said, pilots abide by their (or their company's) rules, which usually entail "bookending" our transmissions, by reading the information back, and ending the reply with the call sign. As far as I know, that's not a required format, but it closes the communication loop nicely.

You say "Yes" but you really mean "No." Read OP's question again; they are asking if there are standard morphological or lexical rules, or if stock phrases form the extent of the language. You list out examples of stock phraseology but this does not constitute a formal "language" structure.

I did read the OP's question, and I stand by my answer. Controllers are given specific instructions in the 7110 on how to issue clearances and control information. Less formality is allowed when obtaining information. The stock phraseology must be used in accordance with 4-2-1 (which make up our syntactic order and grammatical dependencies), or it's scattershot. You and I have both heard bad phraseology with clearance items in the wrong order. Doesn't it hurt your ears?

Supplementing other answers: there is no formal language for ATC communications, although it has been discussed, and several tasks in contracts have attempted to make recommendations.  The language for ATC has its origins in railroad communications.

Aren’t FAA ORDER JO 7110.65Y and the Pilot/Controller Glossary a type of formalization of ATC language?

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Why do military aircraft use a different kind of aviation fuel?
Regular airliners use JET A-1 fuel. However, I've seen that military aircraft, i.e., Airforce or Navy; use a different kind of fuel called JP-N, where N is a number.
What is the reason why this happens? If those other fuels have different properties, which are them to be better than the JET A-1 fuel properties?

One reason is convenience and availability. They use JP-8 in ground vehicles, tanks, generators and heaters. One fuel for everything

JP-5 is also easier (safer) to handle than JET A because of its higher flash point.

Please note that additives in JP-8 are toxic.

Note that US DOD is converting from JP-8 to Jet A.

Military aircraft can and do fill up on civil Jet-A -- this is common at facilities where the extra expense of providing both Jet-A and JP-5 or JP-8 fuel isn't worthwhile.  However, the military jet fuel types do provide some advantages over Jet-A:

JP-5 has a high flash point, improving its handling properties over civilian jet fuels.  This is important for fire safety in the close confines of an aircraft carrier.
JP-8 is similar to civil jet fuel, but also has extra thermal stability additives in it.  This allows for longer-term storage under harsh field conditions, and better resistance to deposit formation in places like afterburners and fueldraulic movable nozzles where the fuel is exposed to high temperatures even before it's burned.  It also may be specified for properties such as cetane rating that are essential for proper performance of diesel reciprocating engines run from JP-8.

(I'm omitting JP-4 from this discussion as it's a wide-cut fuel, comparable to civil Jet-B, and was retired from USAF service in 1995.)

JP-4 is also no longer used.

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Does lift coefficient vary with the wind velocity for a given angle of attack?
I have this question in my mind. I have been trying to look for this in different books about aerodynamics and I couldn't find the answer. I know I could check the answer experimentally, if I had a wind tunnel or some advanced software, but I don't have access to any of them.
Let's say we have this plot (fig. 1) obtained experimentally by testing an airfoil at a low speed (subsonic) with a wind tunnel. If I were to change the wind speed of the wind tunnel, would the graph change or would it remain the same?
For instance, if the wind velocity was initially 15 m/s and then 25 m/s, would it change or is the CL-AoA relationship constant for a given airfoil?
I just need to understand this, I don't need any example with numbers (I just give the numbers to have an order of magnitude of the velocities of the testing. And assume the airfoil has a regular shape, for instance a NACA0012, which is pretty common for academic examples.

Any wind will change the change the velocity and the angle of attack.  Always remember that the angle of attack is the angle of the aerofoil to the relative airflow, not the pitch angle.  I'm not sure that I understand your question.

I was meaning that if we somehow fix the angle of attack and the wind speed increases, what happens with the lift coefficient? Now that you told about the pitch angle maybe it doesn't have sense what I asked, but I'm still not sure. @Simon

@Airman01 Lift increases as the airspeed (and therefore dynamic pressure) increases, but the Coefficient remains the same. The graph is a constant, as the Coefficient is there to quantify all of the complex properties of the wing that can't be mathematically quantified

Yes, it does vary slightly due to viscous effects.
In inviscid flow, the flow speed would not affect the lift coefficient - angle of attack relation. However, increasing the flow speed will result in a thinner boundary layer and a slightly different shape of the airfoil - boundary layer combination as "seen" by the outer flow. This influence is captured by the Reynolds number. See below for a plot of the venerable NACA 4412 from Abbott and Doenhoffs collection of airfoil data (picture source):

Note that the lift coefficient is plotted for Reynolds numbers R of 3, 6 and 9 million.


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What is (are) the accidental cause(s) most feared by the flight crew?
As the question says, which is the accidental cause most feared (in general) by the flight crews and what's the reason of it?
Apart from hijacking, pilot incapacity or that kind of issues, I am referring to causes like aircraft systems failure, fire on cabin, etc.
Observation: I'm not asking this question to be an opinion question, since I want to know in general, what is this cause and reason(s) to be the most feared accidental cause.

I doubt that you can get an answer that does not involve some opinion since flight crew members vary widely as to what they fear, which of course is true of the general population. Also, answers would, in my opinion, also vary greatly depending on level of experience, type of equipment, where in the world you were operating, who or what you were carrying, etc. Just one example, when I was flying freight that was loaded in third world countries, my major concern was how badly underweighed it had been and how well it was secured. It will be interesting to see the answers you get.

Not being facetious, but the #1 most feared accident cause is crew error. I (as a crewchief) have muttered, and have heard my pilots say something to the effect of "Dear God, don't let me screw up."  Anecdotal, yes, but the question begs for anecdotal answers.

Are you including external causes, e.g. windshear at landing, or just looking for causes which origin is inside the aircraft, e.g. failures?

@mins I'm also including aerodynamic and meteo issues like those you've said.

I sincerely do not see how this can possibly be not opinion based, unless there has been a systematic poll among all crews (commercial, military, private) around the world.

Is it not possible to objectively discuss the opinions of a large troup of people? Individually, "I like Brand A" and "I like Brand B" are just opinions, but "Brand A has greater market share among this group for these reasons" becomes a more factual & objective discussion.  To the extent that this question is NOT a poll of what each individual is most concerned with, I would suggest it meets the intent of the rules & is a worthwhile topic. JMHO.

@RalphJ that's the point, how do you check the "market share" of the different fears? has someone ever polled worldwide flight crews about that? how often? was it an internet poll or a scientifically accurate one?

My data isn't quantitative, but is based on having worked with, trained with, and talked with hundreds of military and airline pilots over the course of a couple of decades.  Do I have a handle on what things concern pilots more than others? Even without scientific polling, I'd suggest that I do. I'll let the community here make the call.

What crews should be most concerned with is indeed crew error.  In the 1930's and 1940's, more accidents were caused by mechanical failures -- things failed that made the airplane unrecoverable.  Several decades ago, crew errors became the larger source of accidents, where a flyable aircraft (albeit with perhaps some issue, but flyable nevertheless) crashed because the crew did something that made the situation worse.  The hardware these days is pretty darned reliable, and if "you" do what you're supposed to do, chances are really excellent that you can get it on the ground safely.
That said, pilot psychology plays a major role in answering the question, since pilots strongly prefer to be "in control."  Or, less formally, we all tend to be control freaks.  (Not necessarily in every area of our lives, but certainly in the cockpit, we tend to have "take charge" personalities.  If something isn't right, we want to make it right, and a laissez faire "eh, whatever" attitude doesn't tend to be found among pilots at work.)
So the things that most concern us at a more visceral level, I'll suggest, tend to be the things that we can NOT control.  If an engine catches fire, I'm confident I can put it out with the procedures and tools that I have, and I can get the aircraft back on the ground because I'm trained to do that even with one less engine than usual.  But if there is a raging fire someplace that I can't control (electrical wires -- SwissAir Flight 111 or cargo hold -- ValueJet Flight 592), then that bothers me.  Similarly, Terry's scenario of cargo loading done wrong -- outside my control, and the pilots have to trust that somebody else did their job correctly.  And by the nature of that being entirely outside my control, that tends to be more of a troubling scenario than any of the typical list of things that I can control and have been trained for: engine fire, depressurization, electrical issues, etc.

I have also heard the uncontrollable fire as being what pilots fear the most a lot.  Especially in the middle of the Atlantic (or Pacific).  With lithium ion batteries, it could really happen at any point in the flight too.

This answer contains a lot of interesting points, but does not answer the question which is the accidental cause most feared (in general) by the flight crews? and rather what **could be** the accidental cause most feared (in general) by the flight crews? and hence is speculation.

@Federico Feel free to edit, if you want to make it more appropriate! Thank you.


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Vapor trail disappears then reappears
I was watching a commercial jet and the vapor trail was moving and disappearing very quickly. I should note that it is currently 61 F on the ground, but the morning has been cool. (Min 34 F) I don't know what vapor trails do under certain conditions, but I would assume that in warmer weather the vapor trail would completely disappear more quickly, but that doesn't seem to be the case in the summer. Anyway, the weird part was when the vapor trail completely disappeared or cut off. The plane stopped producing a trail for about 15 seconds and the plane was hard to see during this period. After that the trail reappeared and resumed as normal. I also noticed that the trail hung around much longer and dispersed more than all the other trails from planes that recently flew by. Perhaps the plane entered an area of air that was different in temperature than the air around it. Has anyone seen or heard of this and have any idea why this would happen?
Thanks

Hi.  Welcome to the site.  It just passed through a mass of air not moist enough for condensation to form. As to why it hung around for longer, the wind at it's altitude was weaker than at other levels.  I don't know why you were downvoted, no valid reason I can see, so I've reversed it and then some with a +1.

Simon is right its just not moist enough. Like somedays it rains and somedays doesn't just depends on the moisture in the air.


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Lateral location of the center of pressure
I am working on a simple flight simulator. For this, I need to calculate aerodynamic forces and apply them to the aircraft. As I understand it, lift and drag act through the wings center of pressure.
Unfortunately, all the information that I have found so far only talks about the location of the center of pressure in relation to the chord and the airfoil. Something like this:

So my question is, where would the CoP be located along the length of the wing? Say I model the aircraft as having two main wings, where would the lift force be applied? Would it be in the center of wing? I know that the location forward or aft on the wing depends on the airfoil.

Center of pressure is more a mathematical construct. Aerodynamic center has a more practical use. Normally it lies in the middle of the wing due to symmetry.

What is being referred to in the 3rd dimension is lift distribution.
This has several practical applications in wing design because torque stress on the main wing spar increases with distance from the center of gravity if 2 dimensional lift remains the same along the entire wing.
Some choose to taper the wing, as sea gulls do, to lessen the strain on the wing during, for example, a high G maneuver.
Another clever way is to "wash out" the wing tips a few degrees, which, along with a little sweep, creates very desirable pitch stability up to and including stall, as well as creating the sought after "elliptical lift distribution".
Both wing torques should be identical (with respect to the center of gravity).  Aileron controls create an imbalance, causing the aircraft to roll.

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Why isnt the structure of the aircraft affected by high true air speeds or high ground speeds?
Let's suppose that in Cruise flight I exceed the VNE in TAS or in ground speed, why isn't the structure of the aircraft affected by the high speeds?
I am sorry for my english, I am not a native speaker.

When the speed in the air is within limits, how do you imagine that the speed over the ground could cause problems with the structure of the aircraft?

Suggestion: pick one or the other to ask about, TAS or ground speed.  They are two totally different things.  Or post two different questions.

See "dynamic pressure".  They talk about that with rockets too.  Ground speed does not matter because the plane speed is relative to the airmass, but True Airspeed, relative to Mach number, can have stress effects.  Read on.

All aerodynamic forces scale with the dynamic pressure ($q=0.5\,\rho\,V^2$).  This is true for lift and drag -- hence the use of lift coefficient ($C_L=L/(q\,S)$.  This is also true for pressure coefficient ($C_p=(P-P\infty)/q$).
Here, $V$ is the true airspeed.
While we're at it, the equivalent airspeed is essentially a measure of dynamic pressure.
When you are at altitude, the density ($\rho$) reduces, so to get to the same dynamic pressure, you go much faster.  For the same airplane - at the same angle of attack - to get the same forces, you have to go faster.
So, structural loads for an airplane are all in terms of equivalent or indicated airspeed.  A V-n diagram used to express the load envelope of an aircraft is generated in terms of equivalent airspeed.
A fighter jet - or a rocket launch, they will talk about $q_{max}$ -- the maximum dynamic pressure the structure can take.
As far as groundspeed -- the only time the aircraft structure cares about ground speed is when you are in contact with it.  That happens during takeoff, landing, and crashes.

Are crashes not a subset of landings?

@Frog perhaps so, but the role of ground speed in them is related, but different.

why isn't the structure of the aircraft affected by the high speeds?

As also explained in this answer, Velocity Never Exceed (Vne) is defined as 90% of the Velocity Dive (Vd) which is the highest speed after which structural damages do occur. The 90% is given as a safety margin and it is the speed at which the "red range" begins. So theoretically at speeds between Vne and Vd no structural damages should happen.

Let's suppose that in Cruise flight I exceed the VNE in TAS or in ground speed

Please note that all those speeds are given as Indicated Air Speed (IAS) and not in True Air Speed (TAS) nor in ground speed. This answer gives a good overview about TAS and IAS.


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How to approximate the drag for rectangular vertical flat panel placed along the wind and inclined?

So on the top is the view from the top, below it is the side view.
If the angle is zero it is similar to simple flat airfoil.
So I calculated the Cl and Cd in xfoil and
got the Cl = 0 and Cd = 0.02398 :

It is for 30 cm length in 5 m/s stream
Cl will be directed to the viewer, perpendicular to the panel.
Was it a correct application of xfoil ?
How to approximate the Cd also for non-zero angles like 1 ~ 15 ?
Will it be "almost the same" so the difference can be ignored ?
At non zero angle the plane of the plate will continue to be parallel to the wind but its geometry becomes more complex, similar to swept wing.

I'd simply decompose $V_{wind}$ in its two components parallel and perpendicular to the plate and calculate the two relevant $C_d$s

@sophit Do you know some book or article describing this decomposition approach ?

? This is basic phisics/trigonometry: your velocity can be decomposed as the sum of two mutually perpendicular velocities, one parallel to the length of the plate and with a value $V_{wind}cos(\alpha)$ and one perpendicular to it with value $V_{wind}sin(\alpha)$. Each one of these two speed generates a relevant drag term, one parallel and one perpendicular, which you can afterward combine in one single term.

Related question and answers here.

@sophit I use V when I calculate the Re to get the Cd and when I calculate the q ( dynamic pressure ). Should I decompose V in both cases ?

@Robert DiGiovanni  It is not a wing , it is more like stabilizer because it is in the vertical plane , parallel to the wind direction


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How to build a rocket?
I am trying to build a rocket (a small one for scientific purposes) for the very first time, I've never tried to do that before, and I can say I have almost little to no knowledge about this, so I am seeking some kind of starting kit, besides the YouTube videos that I watched dozens of it to proceed this crazy idea, so where should I look? and where do I start?

Welcome to Aviation.SE. This question is probably too broad to be answered well in a single post, and rocketry isn't really on-topic on this forum, so it's likely that your question will be closed. For a place to get started, though, check out Estes model rockets.

If you've watched dozens of Youtube videos you've probably watched videos of people that make their own motors, their own rockets etc. Do not, I repeat: do not do that as your first project (or 2nd, or 3rd ...). Start with model rockets, learn, work your way up, and ask questions in more suitable forums Also, depending on where you live, it may be very difficult or impossible to get model rocket motors and also be illegal to fly them.

@RalphJ You're probably right, I think I might close it myself, but before I do that, if you have anything you would like to share besides "Estes model rockets", I would appreciate it, like I said I have little to no knowledge so I need to get the theoretical part first before getting to the making of it.

@Raketenolli In addition to suitable forums, are there any books I could use? I want to read more about it before really getting into doing it because YouTube was a bad place to start with, I want to read more about the perfect design, the perfect materials, and the perfect motors...etc

Try the book by G. Harry Stine titled "The Handbook of Model Rocketry." It's a must-read for beginners.

There is no such thing as the perfect rocket. I think the best book to get started is "Make: Rockets" by Mike Westerfield.

You start by buying yourself a small DIY rocket kit made by the Estes Corporation, as sold in hobby shops all across this broad and prosperous land of ours. Then you work your way up from the beginner-level kits to the more complex multi-stage kits with bigger motors in them. From there you join a model rocketry club and learn how to construct the whole rocket from scratch without blowing yourself up.


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Why are A320s around the world reporting so many problems these days?
From Aviation Herald, A320s have reported 6 issues in just last three days.  What are some possible reasons for such bugs ,like AI 405 had a hydraulic leak, and B6-907 had a hydraulic failure. Are the airline engineering teams to be blamed for such incidents?

Closing as primarily opinion based since numbers like this are meaningless without including what size is the total fleet, how many hours, MTBF and so on.  BTW, hydraulic leaks are almost routine.  Some aircraft are permanent flying hydraulic leaks.

see also: http://aviation.stackexchange.com/questions/19026

It simply follows from the fundamental laws of nature that if you have independent, random incidents with uniform probability, the intervals between them will have exponential distribution, so it is absolutely normal that there are times when there is no incident and times when there are many in short time. You can't make any conclusions from that.

A statement like "model X has reported Y failures in Z days" is not relevant in aviation. In aviation what is relevant is the mean time between failures [meaning the same type of failure] (in short MTBF): it is the mean time, fleet-wise, between two failures (example, two engine failure concerning the same component). The relevant part, is the "fleet-wise": if there would be only 1 A320 in operation, 2 issues a day would bee too many indeed, with hundreds (thousands?) of A320s in service, flying hundreds of hours each day all over the world, I would say that 2 issues a day is a quite decent record.

It is indeed thousands, by the way. 6,700 A320s have been delivered. Since deliveries only started in 1988, it's safe to assume the vast majority are still flying. Of civilian jet airliners, I think only the 737 has more deliveries (at about 8,600.)


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What has been the maximum number of failed landing attempts for any commercial flight yet?

Top DGCA officials say they do not recall any other instance of this kind where a plane tried to land unsuccessfully six times and then finally touched down in the seventh attempt — whether in India or abroad. (Source)

A better written report of the incident.
Coming to the question, what has been the maximum number of failed landing attempts for any fully loaded commercial airliner?

possible duplicate of Multiple go-arounds on the same flight in commercial aviation?

To be fair - it was three attempts at the first field, then a divert for weather, followed by further attempts at the second field which also had bad weather.  But still probably hard to beat, except deliberately...

@aeroalias Hmm... While this flight is addressed in the other question, I'm not sure that it definitively answers the specific question posed here.

@reirab I'm not sure the trivia answer ("How many, what's the record?") is particularly relevant to aviation (and if it were a record someone wanted to have for some reason I suppose the pilot could just keep rejecting the landing and going around until they beat the number, or the passengers rioted over missing their connecting flights).

I don't think this is a duplicate at all. The other question is "do multiple go-arounds happen? tell me some stuff about them" and is quite vague IMO. This is a much more specific (and therefore better-quality) question.

@voretaq7 In commercial flights, pilots don't go around on a whim.

As far as I know, nobody actually maintains a register of go around. A go around is not a failure, nor a fault.
However each missed approach reduces the fuel quantity which may become insufficient. The crew will not continue to try to land at the same airport if the likelihood of better conditions at the alternate airport are high.

This doesn't really answer the question. Although no one may be tracking every go-around, if a flight has 6 or more, there is probably something going on that someone will take note of.

@fooot. Yes and no... Cargo aircraft in hard weather conditions (Nordic countries) or Russian cargo aircraft in Africa may accumulate GA and no one talking about them. Maybe the question should be limited to passenger flights and more than 100 seats.


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Is it possible to design an aircraft as big as A380 with 2 engines?
As it is generally seen, 2 engines are more efficient than 4 engines. So, can we have so powerful engines to design an aircraft as big as A380 with 2 engines?

Difficult to understand the logic behind closing this answer and providing a link to another closed question. Furthermore the other question doesn't really provide the answer. VTR

@mins The reasons are the same, just a little more extreme with an A380 than a 747. Dupe closing seems reasonable to me. If anything, the other one could be re-opened. It should also be noted that OP was one of the dupe close votes.


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Would it be possible to economically repair 9V-SWB?

Damage suffered by the right wing of 9V-SWB, a Boeing B777-300.
The complete incident details could be found here.
My question is, would it be possible to repair this aircraft economically, so that it could be reused in future and the repairing costs get recovered from future options?

We don't have nearly enough information, especially at this early stage, to be able to determine the extent of the damage.

As far as I know, the damage has been categorized as "substantial", meaning that it will be costly, but the aircraft has not been written off.
I would expect that the wing will be completely replaced (thus it will not be cheap), but given the current status anything more is speculation (and what I wrote already is).

Update: the aircraft was returned to service on 12 Nov 2016.


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What would happen if thrust reversal is accidentally deployed during a flight?
Is it possible to deploy the thrust reversal, in the cruise phase of a flight? And, if they are accidentally deployed, what could be the possible consequences?
As the cruise phase requires a lot of lift, so certainly the impact would not be very desirable..

If the aircraft is at cruise altitude and speed, the wing will see a large loss of lift, and will become uncontrollable.
It used to be possible to deploy thrust reversers in Boeings. It was not feared as a problem, since early tests showed that accidental deployments were survivable and the plane would stay controllable. However, these tests were only performed at low speeds, since low-speed deployments were assumed to be the most critical (source).
But then in 1991 Lauda Air 004 crashed mid-flight in Thailand after an uncommanded thrust reverser deployment. After this crash, additional safety features such as mechanical positive locks were mandated to prevent thrust reverser deployment in flight.
I'm not sure about Airbus, but I guess all current aircraft have some kind of lock.
Because engines on modern aircraft are so closely attached to the wing, the thrust reverser exhaust flow is right in front of the wing. Therefore, the wing will see a lower flow speed which results in a loss of lift.


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Why is IndiGo adding A320NEOs more aggressively than any other airline?
Currently there are about 45 NEOs with PW engines and 6E operates 17 of them. These PW engines have notoriously reported many engine issues and 48 of them have been replaced so far. IndiGo itself has faced 6 engine replacements so far.
Usually, we see that when an Aircraft type reports so many problems, airlines usually halt deliveries until the manufacturers fix them. Even with A320Neo, Qatar Airways has cancelled their order for PW NEOs and Lufthansa is deferring the deliveries.
Why is it then IndiGo is adding them so aggressively? It plans to add 4 NEOs in March 2017 only.

This is speculation on the part of a business transaction, so I'm not sure it has a factual answer based on publicly available information. Airbus may have offered them a discount to pick up cancelled orders or extended warranty terms. As far as I know, IndiGo hasn't published a reason.


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Can we use "stand" as a verb in aviation?
Is it correct to use "stand" as a verb and as an equivalent verb to "park" in the context of an airport?

I've only ever heard it used as a noun.  You would never say "Stand your plane over there".  You'd say park it over by the xxxx stand.

Would it be possible to say: "The aircraft will stand at the gate until its departure time"?

I suppose you could. I'd just say "remain at the gate". If you said "stand at the gate" I'd understand it though.

One "possibly" could say all sorts of things and people could probably figure out what was meant, but I agree, that is simply not the way the word is used, at least in US aviation. Aircraft do not stand; they park, they sit, they remain, they stop, and sometimes they even go (ideally, close to on-time!), but I don't think I've ever heard of one that "stands" someplace.


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Why some airspaces don't have high-altitude IFR airways?
I noticed on enroute charts that some airspaces (mainly over Balnkans - e.g. LWSS, LYBA, LDZO, LOVV etc) do not have high-altitude IFR airways. Unfortunately searching online didn't reveal much. What could be the reason for that?
Thank you in advance!

The areas without routes are the free route airspace of the FAB-CE (functional airspace block Central Europe).
In this airspace, IFR flightplans are not bound to airways and routes can be planned to best suit the needs of the aircraft operators.

Thank you very much!

Further reading: https://www.skybrary.aero/articles/free-route-airspace-fra


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Has anyone been diagnosed with PTSD and been able to get a first class medical?
Any input on getting a medical with PTSD is appreciated


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Pressure scanner for multi-hole Pitot-static probe going to 100km altitude and M3.5
I am wondering if anyone has experience in looking for multi-channel differential pressure scanners for use with pitot static probes in aerospace? What companies/manufacturers/product familes would you recommend, as I am struggling to find any that meet all my requirements. My application requires differential pressure sensing up to 30 kPa of 7 pressure ports and absolute sensing of one port up to 1bar with accuracies <= 0.1% FS. As the max altitude is 100km it must be able to output differential measurements in essentially a vacuum (0.5kPa static pressure). As far as I can tell this requires the use of a true differential scanner, as absolute scanners generally have a lower pressure limit.
Space and weight are a concern (ideally <<250 grams), hence why I am opting to look for multi-channel pressure scanners rather than individual pressure sensors. These are the companies I have found so far that almost meet my requirements but not quite:

Chell Instruments NanoDAQ-LTS model: This is a 16 channel absolute pressure scanner with great size, weight, accuracy values. However, lower pressure limit of 12 kPa, so can only work up to 15km altitude.
EvoScann P8-D: 8 channel true differential model with good accuracy, weight, and space, but only rated up to 20 kPa differential pressure.
Microscanner 16MS: 16 channel. Unsure if this is true differential or absolute, but seems to require a data system to interface with (big and bulky, dont meet weight requirement)
Texense 8xPDIF-S: 8 channel true differential, but ±0.5% FS accuracy, ±0.7% hysteresis, ±0.5% drift. So not accurate enough.
Keller series PD-10LX: Single differential pressure sensor model. good accuracy and reasonable size but accuracy is only rated down to 10 kPa line pressure.

Any extra manufacturers or products I might not have considered would be greatly appreciated! Or if you have worked a similar problem before and think that it is only solvable if I relax a requirement (e.g. size, weight, accuracy, pressure limit etc) please let me know.


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What is the approximate lifespan of H125 helicopter
What is the lifespan of the Airbus H125 helicopter used by law enforcement agencies?

Classic lawyer answer: Well, it depends....
Airframes are typically rated on fatigue life for several thousand hours of flight time.  But that is the airframe only;  there are a lot of life limited parts eg rotor blades, which must be periodically replaced.  With a proper maintenance schedule observed, time in service can be extended quite a lot, possibly indefinitely.
Length of service would also depend upon a number of other factors, including, but not limited to:

Total Time logged on the airframe annually.
Exactly what type of flying the aircraft was subjected to ie an H125 will probably have a longer fleet life flying executive shuttle flights vs being used for logging, utilities work or flight instruction.
Environment that the aircraft is operated and stored in (an aircraft which is always hangered after use and flown in a dry, arid place will fare much better than one operated in the tropics and parked out in the elements while not in use.).
Care shown to the aircraft by crewmembers during use (are you leaving pens and other junk in the cockpit?).
Discipline and attention to detail in a required maintenance schedule (have you been neglecting ADs, SBs or preventative maintenance?).

UPDATE:  As the OP has now noted that the intended use of the H125 is as a LE helicopter, it does narrow the conditions down a little bit more. Provided again that it received good care, and most LE agencies, maintain hangers and decent maintenance schedules on their aircraft, it should remain in service with the department, or be replaced by the department, well before its airworthiness, can no longer be maintained.

Fully agree, see my comment on the other answer providing an example for Germany, where law enforcement and emergency services helicopters had to be changed because of noise pollution regulation, not any restrictions on airworthiness.


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Most Efficient Glide: Pitch Up or Level Flight to Bleed Airspeed
This is purely a thought experiment / theoretical question, not asking about practical engine out or best glide procedures.
Say you are cruising at 150 knots and suddenly lose all power. You best glide (V-glide) speed is 75 knots (the numbers don't matter). Would you glide further (straight ahead, no turns) if you:
a) Pitch up to quickly drop your airspeed to 75, then immediately pitch to maintain V-glide (gaining altitude in the process), or
b) Hold level flight until your airspeed bleeds off, then immediately pitch to maintain V-glide (gaining more forward distance in the process), or
c) Some other maneuver?
I believe I've seen an answer to this before but I cannot find it :/ Apologies if this is a re-post.

Not a pilot, but I've always read to pitch up up best glide speed. It makes sense, in that the longer you are not at best glide speed, the shorter your glide will be.

In small one engine planes I have flown (Piper Cherokee, Cessna 172, ... ) the standard procedure is to trade speed for height. Height gives you more options in where you want to aim for your crash landing, and only slightly decreases the glide distance.

Once you lose all power, you become, in the first approximation, a ballistic projectile.
More accurately, you're aeroballistic, but the rules remain similar. The longest-range trajectory will be one where you first trade KE for PE (potential energy), then PE for KE, at an equal rate, resulting in a roughly-symmetrical ballistic arc. However, due to drag, it's isochronously symmetrical, i.e. arc segments take equal time rather than distance.
Such an arc looks roughly as follows:

With a difference that alpha will be determined by your aerodynamics. Overall, your goal will be to lose as little total energy as possible at any point in time. For an artillery shell, the optimal AoA is zero, and alpha is 45 degrees.
For a real airplane, the calculation will be somewhat complex, but comes down to maintaining the best possible lift-to-drag ratio at all times.
The simplest rule is: while above your climb speed, climb, then glide, while minimizing drag. Stick to your best climb configuration until halfway between your best-glide and best-climb speed, and gradually transition to your best-glide configuration at that point.

Ballistic projectile is patently false.

I think ”ballistic projectile” is wrong by definition. An aircraft that loses its power is very much steerable and generates plenty of lift - that is the whole point of this question.

A ballistic projectile can be controllable. I've edited to clarify that this is a first approximation. The second is aeroballistic. But your flight is still governed by energy.

@Therac a controllable projectile isn't in ballistic flight. Obviously every "ballistic" missile has some non-ballistic period of flight, otherwise they wouldn't go anywhere (e.g. an ICBM has an initial burn period), but "ballistic" refers to the non-controllable portion of flight.

"The longest-range trajectory will be one where you first trade KE for PE (potential energy)"
Cite? The optimum trajectory for a ballistic trajectory is based on being, well, ballistic. In a ballistic trajectory, you don't have any choice as to whether KE is being traded for PE or vice versa. The "optimum" is assuming you have a choice in initial angle, and finding what the best choice there is.

@Acccumulation I don't think they even make non-controllable ballistic missiles anymore. Not for the big jobs. Being ballistic isn't about the lack of control, it's about flight governed by expending initial kinetic energy.

Somewhere between a) and  b).
An aircraft best converts its kinetic energy into lift when the wing is at optimal angle of attack.  Here the lift to drag ratio is highest.
If one is cruising at 150 knots, the angle of attack will be 1/4 that at 75 knots.  So, bringing the wing to optimal angle of attack for optimal V best glide will cause the aircraft to climb.
However, in a real situation, if the engine went out suddenly, the aircraft would sink, and the pilot, in pitching to Vbg, would initiate a climb from that point, perhaps crossing the original altitude.

pitch of stick/trim tab should less than or equal to stick/trim tab position at Vbg

Where ever Vbg is reached would be the end of the climb.  Attempting to gain more altitude (up to the point of stall) would be a waste of kinetic energy, trading less efficiently for more potential energy which will be needed to accelerate up to and to maintain Vbg.

If the wind were calm, you would glide farther straight ahead if you were to pitch up to turn excess kinetic energy into potential energy. You can even fly (briefly) under stall when rounding off the top of your climb as your stall speed decreases as the wings are unloaded.
If you bleed off the airspeed, then you are losing that energy at a less efficient point of operation.
And if you pull up too hard, you might scrub lots more energy than you realize in the high-gee maneuver.

If the wind isn't calm, then there are certain choices to be taken into account to maximize your glide range, but that's something we glider pilots think about a lot more than normal folx.


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Manufacturers are narrowing seats. Any possibilities for price cut?

To squeeze more profits from each plane, the airline industry in the
  last few years has begun to install extra rows of seats,  leading to a
  crunch in legroom that has sparked tiffs and feuds among frustrated
  passengers. (Source)

.

Airbus has suggested 3-5-3 configuration for A380, making airlines
  have many extra seats.Source

A similar plan is there for the B777. Is it likely that airlines will pass on some of greater earnings to their passengers, in return of the lack of comfort?

Passing savings back to the customer? Hahahahaha ;)

I'm voting to close this question as off-topic because this is an Airline Policy based question regarding the financial strategies and ambitions of carriers, and hence this question is not about aviation in general.

It could be theorized that it's the already low prices that is 'forcing' the airlines to stuff more in each flight...

You'll probably get a better response if you ask this question on travel.SE instead

You will likely not see a price cut, but you also might not see a price increase.
Seat pricing (along with just about any other service in a competitive market) is a complex function of actual cost to the company + desired profit + market factor. The last one is the difference between what they want to get and what the market is willing to pay. It is sometimes negative.


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Why does a slight elevation on the wings of a paper plane makes it go farther?
See the images of several of the best paper-planes. Notice that their wings are slightly elevated, than a normal paper-plane. That is just what happens with me as well, a little elevation on the wings of a paper-plane makes it go faster and farther. Why does this happens?

please define what you mean by "elevated"

see this image and notice the slight elevation in wings.

elevation, noun: the height of a place above the level of the sea you mean that the photo has been taken on top of Mt. Everest?

@Federico I just thought that you have that much common sense :).

OK, seriously though, are we talking about the "trimming" of the trailing edge of the wings, or are we talking about the "dihedral" of the entire wing angling up from the fuselage? There are different reasons that both contribute to stability.

Most paper planes are nose-heavy and for a very good reason. Basic aerodynamics tells us that putting the centre of gravity forward of the centre of lift, makes any plane (paper or not) more longitudinally stable.
The trade-off is that this configuration tends to cause a nosedive. The trailing-edge elevation counteracts this.
Note that, for the very same reason, all conventional-configuration self-stable aircraft are designed with negative tail-lift.

Aside
A not unheard-of cause of air accident for small commercial planes is
  pilots ignoring the loading specifications in order to please a
  customer. If the positioning of the load puts the C of G too far back, the plane becomes unstable. This may result in an unrecoverable spin.

The aside is counter to the question. Moving the CG forward makes it more stable (but hard to land bc of inability to flare). Moving the CG back makes it more unstable, making stall recovery difficult to impossible.  Adding elevator doesn't changed the CG. It changes the center of lift.

I said that elevator counteracts the tendency to nosedive. I didn't anywhere claim it changed the CG. Not, that is,  unless the baggage is so loosely stowed that it is sliding about. I've reworded that paragraph slightly to be clearer.

Yes much better. Although the instability applies to any stall not just s spin


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Do Freon gas leaks occur in Airplane conditioning systems as well?
I was just reading about Aircraft Conditiong Systems and found that, the A.C. mechanism begins with compressor in the engine. Do these compressor also work the way in which AC/Refrigerator's compressor do? If they do, then Freon Gas Leaks should be something happening atleast once in a long period .. They are pretty common with household Air conditioning system.So, does this happens? I have never really heard of something like this happening to an aircraft..

there is no freon on board of an aircraft. related: http://aviation.stackexchange.com/questions/5031 http://aviation.stackexchange.com/questions/8535

Large aircraft use air conditioning packs (air-cycle machines) which uses only air, compressors and heat exchangers to cool/heat the air. There is no freon involved.

Just read the link. Sent me wondering on the totally off-topic though of "why don't we use these in cars?". Yes, this is OT, no, I don't expect anyone to address it - just thinking out loud.

@FreeMan: The air-to-air heat exchanger works the same way as our freon/ammonia air conditioner - it's a heat pump. We use freon to miniaturize the heat pump.

@FreeMan With a few notable exceptions we don't generally have turbine-powered cars with a compressor section to bleed air from, nor a nice cold environment to dump waste heat to (when you want air conditioning to cool your car it's usually hot out), so an air-cycle machine isn't as attractive in a car as it is in a jet up in the flight levels (with sub-freezing air to cool the bleed air to whatever reasonable temperature you desire):)

While transport-category jets tend to use air-cycle machines like ratchet freak mentioned some light airplanes are available with "standard" (freon heat-pump) air conditioners.
These have the same refrigerant-leak problems as your typical car air conditioner as seals age, pipes crack, etc.
These air conditioners are also not particularly popular accessories because they add weight and mechanical complexity to the aircraft (I don't think I've ever seen a plane with one installed in person, though I know a few folks over on the Piper forums who have the factory air conditioning in their planes).


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Why won't you allow someone into the cockpit while on ground?
Various regulatory authorities around the world have framed strict rules prohibiting entry of passengers into cockpit. It makes some sense to not allow anyone into the cockpit (learning from 9/11), while you are flying but when on ground, what is the harm in allowing someone to enter the cockpit? The passengers are frisked thoroughly before being allowed to board the aircraft, and if you are on the ground then there is a lot of security around. So, what are the factors behind not allowing outsiders into cockpit, while on ground?

Visitors in cockpit, four pilots lose licence

I don't agree with the strict enforcement of this rule by the DGCA (but I'm not an airline pilot so I think my opinion doesn't really count on this matter). But there is logic behind the ban - many safety-critical actions are completed before the plane has left the gate. Such actions include calculating takeoff speeds, briefing the departure procedures, and completing various checklists.
Having people in the cockpit could prove to be a big distraction when trying to complete these tasks, particularly when the time is limited anyway such as on a short turnaround.
Making a mistake before leaving the gate can definitely end in disaster. Take Emirates flight 407 in 2009. The crew entered an incorrect takeoff weight into the system. As a result there wasn't enough thrust to rotate and the A340 barely cleared the airport fence.

There are people already coming in and out of the cockpit while the plane is on the ground. I have seen the flight dispatcher, the load master, members of the cabin crew, etc. all enter and leave the cockpit while the passengers were seated or being seated.


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Why do airplanes have to retract landing gear?
Why do all the airplanes have to retract their landing gear, once they reach some specific height? Why can't they simply leave their landing gear deployed all through the flight?

It's important to note that not all aircraft have to retract their gear, as some of them have fixed (non-retractable) landing gear.

The main advantage is that it reduces drag!

In the early days of aviation it was simply easier to have the gear fixed. The landing gear adds drag, but so does a second wing and wire braces. Airplanes generally did not fly very fast or far, so the drag from the landing gear (and that other stuff) was not a big issue. The first design with retractable gear dates to 1911. Since then, retractable gear has been a design decision each airplane must make.
Retracting the gear into the airplane allows for a cleaner form, which reduces drag. However, this is at the expense of added weight. The retract system, which is usually hydraulic, must be added, and the plane must be designed to make room for the gear somewhere at least mostly inside the aircraft. The plane must also be designed to handle situations where some or all of the gear does not extend properly. For planes that need to be fast and/or efficient, retractable gear is worth the reduction in drag.
The additional drag has to be supported by the structure as well. Aircraft with retractable gear will have a maximum speed at which the gear can be extended. Even gear with proper fairings would add significant drag at the speeds airliners fly at. This would be much more critical at supersonic speeds. In these cases the weight of reinforcing the gear could be even higher than the weight of the retracting system.
Particularly smaller general aviation planes tend to not have retractable gear. The retract system adds extra weight and complexity (and therefore cost), which will be fairly significant in a small plane. Fixed gear is simple and can be designed to minimize drag as much as possible. These planes do not typically need to fly fast or have large range, so the added drag is less of an issue. Other small planes do have retractable gear, which will allow for greater speed and range. The PA-32 type shown in Dave's answer was later produced in a retractable gear version as well.

The reasons you mention are, of course, the primary answer. However, it might also be worth noting that the gear on some types is actually not designed to withstand being deployed at the maximum or cruise speeds of the aircraft. As an extreme example, I'd imagine the gear on a fighter would have a bad day at Mach 2 (let alone the gear on the SR-71 at Mach 3.3 or the gear on the Space Shuttle during re-entry.)

@reirab Thanks, the speed limitation is certainly important. I added a paragraph for this.

Also, the gear would have to not burn up in the case of supersonic flight. :) Making a tire that can survive at Mach 2 (let alone Mach 3+) would not be trivial, even if the weight weren't a factor.

According to this chart, given I read it correctly, the gear would go up to about 170°C (340°F) at mach 2. I would have thought it would not be too difficult to have it withstand that temperature?

@spectras Isn't that just the temperature of the air? Friction and vibration would cause extra heat on top of that. In case of a rubber tire that has uneven surface, mediocre aerodynamics, and is specifically designed to absorb vibration... I'd expect there would be quite a lot of that extra heating.

The SR-71 heated to over 300 °C in flight, which is why so much was made of titanium and landing gear would certainly be a challenge in those conditions.

@VilleNiemi from what I get, this is the temperature of the air, taking into account compression heat. I might be wrong, but I read that at supersonic speeds, friction heat is negligible before compression heat, so it should not get much hotter. That seems in line with fooot's figure of 300°C at Mach 3.2. Granted, that's way too hot to be practical :)

@spectras Whether friction is negligible depends on the shape and surface. You are probably right about it being possible in theory, if somebody designs the gear for specifically that. But with the drag issues nobody is ever going to make the effort.

@VilleNiemi: You could, of course, make the tyre out of something other than rubber.

@Sean Sure, and in fact you'd probably have to but I was just pointing out that the properties and shape for a good tire would increase the temperature over the values for normal aerodynamic surfaces spectras was quoting. Regardless of the material.

Short answer: It allows them to fly faster and further.
Long answer: Hapag-Lloyd Flight 3378 demonstrated this impressively on July 12, 2000. Destined for Hannover, they did not retract the gear after take-off in Chania, Crete. The fuel was sufficient for the planned distance plus reserves, but the extended gear increased fuel consumption so much that they ran out of fuel while approaching their diversion airfield near Vienna. Fuel consumption enroute was twice as high as it would had been with the gear retracted.
Drag is composed of a lift-related component (induced drag, blue line below) and a constant component (zero-lift drag, red line below), and both depend on the dynamic pressure, which is the product of air density and the square of airspeed. While the lift-related component goes up with reduced dynamic pressure, the constant component will go up with increased dynamic pressure. Thus, extending the gear will increase the constant drag component, and while flying slower helps to reduce its contribution, it will drive up the lift-related drag component. In the end, the drag will be higher at all speeds.

Typical drag contributions over speed for a glider. The physics for airliners is the same, only the numbers are bigger. In cruise, all airliners try to fly as close to the minimum drag as possible.
The only reason for a fixed gear are cost, weight and simplicity. Performance will always suffer from it.

Maybe good to mention an approximate point at which it pays off. Roskam (Airplane Design: Layout of landing gear and systems) mentions: "Experience indicates that airplanes with cruise speeds above 150 kts tend to use retractable landing gears because of gear drag penalty"

@ROIMaison ... and just about any glider going over 50kts, even if "retractable" kind of overdescribes a wheel peeking out of the fuselage ;-)

You're correct, I should have mentioned that Roskam was focusing on powered aircraft. The decision changes quite a bit if you can make a light and simple retraction system such as in gliders

Because its a drag to leave it down(pun intended). Some smaller GA planes (and big planes too) can extend the landing gear through a wide range of their operating speeds and use them as a pretty effective speed break. The DA42 comes to mind in this regard.
For what its worth there are many general aviation planes that have fixed gear.

Retracting gear in a small plane has always been an interesting topic. Some older piston singles have manual retracting gear like the early Mooney M20's which used a Johnson bar to retract the gear. This some what dated system is, by some very sought after for its simplicity.
(the big silver bar in the middle is the gear lever)

Folding gear also has little to do with aircraft size. The Mooney M-18 "Mite" which is a tiny plane by anyones standards had retractable gear (also Johnson bar operated).

One way to reduce drag on a fixed gear plane is through the use of gear fairings. while they do help with speed they can also cause an issue if the plane touches down hard. In some cases (if the wheel is improperly inflated) a hard touchdown or bounce will cause the wheel to balloon out while also spinning which could chafe or seriously damage the fairing.

Fairings - also known as a cowl or boot.

@KeithS, or spats, or wheel pants! :)

For the last word you used "fearing" instead of "fairing".

You are correct, a change has been made.

Why do all the airplanes have to retract their landing gear, once they reach some specific height?
  Why can't they simply leave their landing gear deployed all through the flight?

Both questions are false on their premise:

All airplanes do not have to retract their gear;  many planes are fixed-gear, and cannot retract gear.  Some planes retract their gear.
Planes can leave their gear deployed all through flight, provided they remain slow enough that the extra airflow drag doesn't damage the airplane and landing gear.

Planes that have retractable gear generally retract their gear for improved performance.  Not having wheels and struts hanging down streamlines the plane, improves fuel efficiency, enables them to fly faster and higher, and makes for a quieter, more comfortable ride inside the cabin.  So, retracting gear is generally desirable.
But, as your question suggested, it is not required, and it is not based on a specific height.  (if anything, it is based on an airspeed)

Simply retractable landing gear is made so as to reduce extra drag which created by the fixed landing gear and hence heavier than air aircraft(airplane) can improve in speed and travel at high speed at higher altitude

I ask if anyone with some questions concerning with any part in aviation,please send to me

Welcome ! What does this comment mean: "Any part in aviation" and "send to me"?

This answer doesn't add new elements compared to the answer selected 7 months ago, right?


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I recently flew from Bar Harbor Maine to Boston Logan airport on a Cessna 402 turbo prop airplane. We didn’t have a copilot is this a FAA violation?
I flew from bar harbor Maine to Boston MA on a Cessna 402 turboprop with one pilot is this a FAA violation?

I cannot find a definitive citation for this specific aircraft, but very likely no.  Many aircraft of this size are certified to operate with a single pilot.

A Cessna 402 is a turbocharged reciprocating twin engined airplane, maybe this is what the OP meant.

The Cessna 402 is a single pilot commuter airplane. It has two sets of controls, but the second pilot seat can even be used as an extra passenger seat.
It is not a turboprop, it's a piston engine airplane, with MTOW less than 12,500 lbs. As such, it can be operated by one pilot with a multi-engine land endorsement, depending on how.
Not always though. As 757toga mentioned, under Part 135 IFR, it would require two pilots.


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1. Why does the glide slope guide you down to 0.7NM away from the runway? 2. Best way to circle from 28 to active runway 36?

Why does the glide slope guide you down to 0.7NM away from the runway? 2. Best way to circle from 28 to active runway 36? Note parachute jump site takes place on SE corner

It would be a better idea to separate this into two questions

I agree, and the question about merging a circling approach with VFR traffic is well covered here:  https://aviation.stackexchange.com/questions/99358/how-is-the-vor-a-approach-at-kvcb-meant-to-be-flown/99366?noredirect=1#comment276939_99366

Also, consider clarifying why you are asking about 0.7nm, is it because you think that is far enough out that at 313' AGL you ought to be able to press in further, break out, and still make a safe landing?  (Because we don't know your background or knowledge level, and the short answer is "because that is the MAP...")

Note the gray triangle on the profile view is not vertical guidance; it indicates there are no obstacles in the visual segment after you reach the MDA (or DA, as appropriate).

First off: it doesn't. The approach has no vertical guidance. Once you've passed JIBDO you can drop all the way to the MDA as quickly as your plane is capable of. Below the MDA there is no guidance at all- you must have sighted the runway environment in order to descend below the MDA.
Also, despite how it is drawn, a constant descent angle of 3.06 degrees from the FAF will get you to the runway at TCH, and won't crash you into the ground 0.7nm short of the runway:
$$ \text{VDA} = \tan^{-1}\left(\frac{\text{FAF altitude-TCH-TDZE}}{\text{distance ZURBU to threshold}}\right)= \tan^{-1}\left(\frac{1953~\rm feet}{36464{~\rm feet}}\right)\approx3.06^\circ$$
It's calculated as though you turn directly towards the runway threshold as soon as you cross the FAF. You wouldn't normally fly it that way (certainly not if you're flying it to minimums!), but using the same angle flying all the way to the MAP and then to the threshold still puts you at a TCH of 31 feet. Not underground as the profile view would have you believe!
The final approach course here is 20 degrees offset from the runway. If the final approach course took you directly to the runway threshold, it wouldn't leave you any room to maneuver to align yourself with the runway. So the final approach course has to intercept the extended centerline of the runway with some additional room for maneuvering. In this case, it is set up 4499 feet from the threshold, a little over 0.7nm.
If you pass through the extended centerline of the runway without sighting it, it's unlikely that you are going to, and it would greatly increase the maneuvering required to land safely even if you did. So the missed approach point is set on the extended runway centerline.

@757toga I guess they can't be coincident if one doesn't exist!

The procedure was made in 2014. It’s likely that there was a PAPI then to RWY 28, which is why the procedure form has that note. Or it’s an error. See the procedure’s 8260-3 form here: https://www.faa.gov/aero_docs/acifp/NDBR/2016062128729904002-X51-NDBR/00A_FL_HOMESTEAD_RG28_X51.pdf


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ATC Phraseology “fly heading” vs “turn [direction] heading”
This may be a UK/CAA phraseology quirk, but I’ve heard controllers use the phrase “fly heading 090” and “turn right/left, heading 090” seemingly interchangeably.
When would a controller use each phrasing?

In general, I use "fly heading" when I'm not 100% certain of an aircraft's heading (e.g. if they're proceeding direct to an airport fix) and there's a chance they might be on the "wrong" side of the heading.
For example, imagine that an aircraft is on a ground track of 290°. I would like to tell them to fly a specific ground track, but the rules don't allow that. I want them to fly heading 300° for now, but I don't know what the winds aloft are doing—they might be on a 280° heading to get their 290° ground track, or they might be 315°. Instead of telling them to turn in a specific direction, I just say "fly heading 300" and let them turn in the shortest direction.
Most of the time, though, I specify turn: left or right. And if it will be the long way around I repeat myself for emphasis, or maybe even say "turn the long way" in so many words.

After getting ADS-B downlinked magnetic heading on our radar, I really enjoy giving the turn direction for very small turns, e.g. "turn left heading 290" when I know they are currently heading 292. I like to think the pilots get a bit confused/impressed with my precision :D

In UK CAA CAP 413 Radiotelephony Manual, Chapter 5 (Phraseology associated with the use of ATS Surveillance Systems), see following sections (page 8):
5.15. Heading instructions may also be combined with a turn instruction e.g. continue turn heading 240 degrees.
5.16 A controller may not know the aircraft's heading but does require the aircraft to fly a particular heading e.g. fly heading 275.

Besides reiterating what's already been stated in answers, I don't see how this provides an actual answer to the stated question.

@RalphJ It's UK specific and provides the relevant citation for UK ATC. I think it's a good answer vs one which is anecdotal and one which is US focused.  The "may also" surely answers the question that it's to the controllers discretion?


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Consequences for passengers taking baggage during evacuation
There are obvious safety reasons to leave luggage behind during an emergency, but many passengers knowingly ignore this and choose to take their luggage with them.
I haven't found a clear answer: Do such passengers actually face consequences for doing so? Are they breaking some kind of law? If not, do they get placed on a no-fly list?
I would be happy with any answer to the question, but I am particularly interested in instances where general luggage collection slows down an evacuation during an emergency in which people die.

I'm sure they say it because they are assuming the plane might catch fire - so every second counts.  Now how many times have you been waiting to get off a plane and someone infront of you has their baggage at or behind you - delaying everyone for ages !!

@MrR That's part of it, but trying to evacuate with luggage carries a high risk of injuries on the slides or stairs to oneself and to other passengers. Dropped or bulky items can also block the evacuation route.

One consequence might be getting knocked to the ground and trampled by those behind you.

Obstructing an evacuation by taking unnecessary items isn't just a breach of airline rules. If there's no fire, everyone gets their luggage back anyway. If there is a fire, taking luggage means prioritizing it over other people's lives.
Willfully endangering the lives of others, including to protect your property, can be a criminal act. This would be considered endangerment. The level of culpability applicable to a passenger is criminal negligence, so under Common Law they can be held criminally responsible only if their actions contribute to someone else's bodily injury.
Example: if someone's suitcase punctures a slide, and the next passenger breaks a leg as a result, that's grounds for an endangerment case. If it delays the evacuation, but everyone gets out alive and well, there's no liability. If not, it can get serious.
There's an understandable reluctance for DA-equivalents to charge people already victimized by the crash. I've asked on law.se to see if there have actually been any cases. Civil liability also applies, which is easier to prove and split between multiple defendants.
While it can be compared to drunk driving, where one also loses their license, plane evacuations are very rare, and there's no separate penalty system like a no-fly list for this.
If no one suffered as a result, in the US, there's no case, but the FAA can fine the passenger for misconduct, with fines far in excess of flight or baggage costs. Not all aviation agencies have such powers, but civil law jurisdictions can have statutory penalties regardless of harm done.

"If there's no fire, everyone gets their luggage back anyway. If there is a fire, taking luggage means prioritizing it over other people's lives." <-- I feel like this should be part of the preflight briefing.

A tangential answer that may at least partially answer it. I happened to come across a very spirited discussion in an aviation video comments section on this topic. It was enlightening and somewhat surprising to hear so many different opinions, and to find out what average, random travelers think about this topic. Which I think could possibly impact what the airlines' policies are on this even though it's a potential safety issue. And it could explain why there isn't a lot of discussion about this from the airlines.
Previously like many I thought it seemed pretty straightforward, and that anyone taking their carry-on or whatever items with them during an evacuation is just a degenerate, unprincipled reprobate who probably should get the book thrown at them. But a surprisingly large percentage felt differently. I almost think it was even a majority. One point made was that it’s not so simple getting your stuff back. Which makes sense when you think it through. They can’t just let people back onto the plane to go back to their seat and get their items. If it was that easy then it would be no problem. Instead it all has to be collected and taken somewhere, and then there is a process of matching people to their items. I have no idea how it’s done but it sounds messy, and it apparently does not necessarily happen the same day.  And it seems quite possible that items will be returned to the wrong person based on misidentification, since this is presumably done through a formal written process, similar to a lost luggage scenario where people describe the item that they are claiming. In fact I suspect that the same airline personnel who handle lost luggage and items left on planes manage these situations, since it's basically the same thing.
From the comments it was clear that there was a lot of distrust of the airlines that they would ever get their stuff back. Justified or not that is what many people felt. A recommendation that was made in the video comments, and even here, is that the airlines should make an announcement of some type reassuring passengers about their items if they are left behind. But since I suspect that the actual situation is not pretty they may be reluctant to do so. An interesting separate question that could be asked is how long does it usually take to get your items back and what is the process, and what percentage of people never get their items back.
A real big issue that was brought up repeatedly was medications. Many people said they would be in danger without their medications, and so had to bring it with them, and because of this they had no intention of following whatever rules there might be.
As relates to the question, I would guess that airlines are aware of these opinions within the travelling public, and they are not likely going to be aggressively pursuing people who violate the rules, because it would not be good PR. And they likely don't want to bring additional attention to an evacuation incident. The only time it would ever be an issue that had to be dealt with is in a worst-case scenario where let’s say the last 25 people did not get out in time, and it had been observed that some people had brought their luggage or small bags with them during the evacuation. However even if the evacuation happened to be caught on video and individuals could be identified, I’m not sure that they could prosecute people who had been seen carrying their luggage off the plane, because it would be nearly impossible to prove that they directly contributed to the deaths of the other passengers. Unless there happened to be say a video of a passenger struggling to get their carry-on out of the overhead bin and holding up the passengers behind them who didn’t make it off the plane.
A really good point that came out of the discussion was that medications should be kept separately, perhaps in a small bag, or maybe even worn in a fanny pack or similar. What also seems clear is that any small carry-on items brought onto the plane should contain some type of ID in them, and preferable on them, perhaps using a luggage tag when possible or a sticker, which includes name, phone number and e-mail. This will likely result in getting the items back much quicker, and lessen the chance of them getting sent to the wrong person. This also could help in other situations for example an item that is accidentally left on the plane or in the terminal.
All that being said, this doesn’t change the fact that each person that takes even five seconds to grab an item creates a multiplied delay, which could potentially be catastrophic and result in unnecessary fatalities. But the odds of this situation actually happening are probably in the millions. There is a chance that luggage being brought down the slide could lead to someone being injured, but this would also be rare and again difficult to prosecute.
I suspect that the situation will remain status quo, and that in the end people will do what they want to in that situation, and they will likely not face any repercussions.

How do you feel about having locks on the overhead bins, activated via an emergency checklist in the cockpit (or just automatically locked then the plane is at, say, 5,000 AGL, and only unlocked when the seatbelt sign is off (and overridden during an emergency)? If someone's meds are that important, they can have paramedics administer them or get them from an ER visit (at the airlines' expense).

I will comment in relation to the question, i.e. what do I expect airlines or regulators to do. I won’t go so far as to say it would be solving a problem that doesn’t exist, but rather one that is very rare. In the U.S. a total of five people have been killed in airline accidents in the past ten years. None of those fatalities had anything to do with evacuation speed. That is out of 79 million domestic passenger flights. The potential risk is always there, but the cost and complexity of that type of solution makes it unlikely that regulators would impose that type of requirement on airlines.


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Why is the helicopter rotor brake easily accessible?
According to this article, a rotor brake is dangerous to use in-flight, yet it's easily accessible and the pilot's reaction indicates that it would've deployed mid-flight. Why is the brake so easily accessible and deployable during flight if it's such a safety hazard?

Why is it easily accessible? Well simply because it's intended to be.
When you consider the main usage of this system, it it to allow the pilot to expedite the slowing down and stopping of the main rotor on the ground, usually to allow safe ingress/egress of the passengers. If it wasn't easily accessible by the pilot it wouldn't serve it's purpose.
There is a question here of whether a helicopter such as this, where it is easily reachable from a passenger seat is a suitable aircraft to take tourists, or anyone untrustworthy around the primary controls of an aircraft. I think sometimes people are just a little dumb around things they don't understand - ie/ I'm almost sure nobody is stupid enough to pull on a car handbrake at highway speeds - but that's primarily because they understand the implications.
In any case, the helicopter engine is likely to have had well enough power to overcome the application of the rotor brake (similar to your car engine being able to overcome the power of the hand brake if youve ever accidentally pulled away with it on). The problem comes if the pilot needed full power at that exact moment they may not have had it all available if stupid tourist pulled the brake.
Pilot conclusion: Make sure you fully brief lay passengers before letting them within reaching distance of primary controls while airborne. You're taught this on day one. The passenger here was dumb, the pilot was negligent!

Thank you. Wouldn't placing the brake on the left side of the pilot be a simple yet vastly safer option?

Or have a 'squat' sensor like fixed-wing aircraft gear?

@AhmedTawfik Both options sounds sensible to me. Does make you wonder why the designer didnt do it, right?

Agreed, hence the question :)

@Jamiec Is the linkage mechanical? If so, perhaps moving it to the left of the pilot would be difficult or impossible?

It's not clear what type of helicopter this is, but bear in mind many helicopter designs are fundamentally still 1960s-80s, fully mechanical, with simply updated avionics and engines. There probably wasn't any consideration for having untrained people in the front seat.


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What do you mean by "high-G climb out"?
In Top Gun Maverick (2022), Vice Admiral Beau "Cyclone" Simpson speaks to squadrons about enemy terrain:

Vice Admiral Beau "Cyclone" Simpson: Well, Lieutenant, you have a fighting chance against enemy aircraft.
What are the odds of surviving a head-on collision with a mountain?
You'll be attacking the target from a higher altitude, level with the north wall.
Gonna be a little harder to keep your lase on target, but you will avoid the high-G climb out.

What do you mean by "high-G climb out"?

Well, technically the pull-up, not the climb out, is where the high G-load is imposed--

What do you mean by "high-G climb out"?

Arguably this terminology is not really correct.  The pull up, where the flight path changes from horizontal (or diving) to a climb, is where the high G-loading is imposed.
G-loading is simply lift force divided by aircraft weight, and a change in direction of the flight path is a form of acceleration, and the rate of acceleration is proportional to the net force acting on the aircraft, so to "bend" the flight path upwards, you have to apply a lift force that exceeds the downward pull of gravity.  Since the centripetal force required to produce a given radius of curvature of the flight path is proportional to airspeed squared, if you are flying very fast and want to bend the flight sharply upward, you'll pull (impose) quite a high G-load.
In the linear portion of the climb, the G-load is actually less than one, because some of the aircraft weight is supported by the Thrust vector rather than the Lift vector.  (For an extreme case, consider a vertical climb.)  For more see this related ASE question -- Does lift equal weight in a climb? -- see various answers including my own.
But if a pilot said "high-G climb out", it would be clear enough to others that he meant a high-G pull-up into a steep climb.
A high positive G-load is experienced by the pilot as an apparent force pushing him/her down into the seat.  This is really the pilot's inertial resistance to the upward push of the seat against his/her body.  A high negative G-load is experienced by the pilot as an apparent force pulling him/her up against the seat belts.  This is really the pilot's inertial resistance to the downward push of the seat belts against his/her body. In both cases the (upward or downward) lift force generated by the wings is the ultimate cause of the sensations experienced by the pilot.  If lift goes to zero, the G-loading goes to zero and the pilot tends to "float" freely in the cockpit.
Actually there's a bit of ambiguity here-- "G-loading" as used in this answer is based on the force component that acts parallel to the lift vector from the wings, which will be generally "up" or "down" in the pilot's reference frame.  The traditional mechanical G-meter on an aircraft's instrument panel measures this force component, or very nearly so.  Arguably the most accurate definition of G-loading would be based on the complete three-dimensional force vector generated by the aircraft.  For example, to truly experience "weightlessness", not only must Lift be brought to zero, but Thrust must be set to exactly equal Drag, so that there is no net force generated by the aircraft, and the pilot experiences no "forwards" or "aftwards" pressure on his or her body.  A catapult launch from an aircraft carrier is an example of a situation where there is a high G-load in the fore-and-aft direction rather than the up-and-down direction, in the aircraft's reference frame. In a steady-state (constant airspeed) vertical climb or dive-- not something most of us encounter in flight, but possible in some aircraft-- the complete 3-dimensional G-loading is 1, but a traditional mechanical G-meter on the instrument panel will read 0, because the wings are not generating any lift.  In this case the pilot feels no pressure in the "upwards" or "downwards" direction in the aircraft's reference frame, but experiences the back of the seat pushing "forward" on his or her back, or experiences the seat belts pushing "backwards" against his or her chest.  The aircraft's excess Thrust or excess Drag vector is the root cause of this sensation.
But when pilots speak of a "high G" situation without adding further qualifiers, they are invariably referring to a high G-load acting in the upward direction in the aircraft's reference frame, which is always due to a strong Lift force generated by the wings.
You will note from other answers that the G-loading can also be defined in terms of acceleration.  Force and acceleration are intimately related.  When considering these concepts, keep in mind that the force exerted by gravity, or the acceleration component due to gravity, is not considered when computing the G-loading.  That's why we don't have to leave the earth's gravitational field to experience 0-G "weightlessness" -- we simply have to stop resisting gravity, and fall freely.

"But if a pilot said "high-G climb out", it would be clear enough to others that he meant a high-G pull-up into a steep climb."
... but the other pilots would laugh, and give him a new tactical call-sign!

I think the best definition of G-loading is the net non-gravitational force acting on the a/c, divided by weight. Which means the vector sum of the aerodynamic and thrust forces generated by the aircraft, divided by weight. The panel-mounted G-meter only gives an approximation as it only measures in 1 axis, "up-down" in the a/c's ref frame. It is accurate whenever thrust and drag are exactly equal, and that's a reasonable approximation much of the time, but not always. But the up-down component is what the pilot cares about, because those are the forces that may break the wings off etc.

The up-down component of the G-loading (in the aircraft's reference frame) is also the component that affects the stall speed.  The fore-and-aft component never gets large enough to break anything, unless you are impacting the ground. The left-right component is rarely a concern either, although if for some weird reason the pilot decides to abruptly apply full rudder deflection at Vne, he/she might get to discover the max G-load the rudder/fin can generate before departing the aircraft--

since gravitational force is a fictitious inertial force, that we cannot feel, you might simply say all real, non-inertial forces divided by aircraft mass-weight.

@CharlesBretana -- point well taken, but I'm not sure that most readers would be on the same wavelength-- so some extra explanation would be required--

Yes, you're right... one of my pet issues, I confess, as I think understanding this actually simplifies so much by eliminating the need fir convoluted wrong explanations we have fed student pilots fir 50 years.

The aircraft is in a dive while on attack. Once the payload is dropped, the aircraft must exit the target zone safely. The aircraft has to pitch up violently in order to avoid hitting the ground and the rising mountainous terrain in the flight path. The speed of the aircraft combined with the rapidly changing pitch angle causes a “high-g” load on the aircraft so that it can “climb-out” to a safe altitude clear of any obstacles.